Document: MIC-3-58-22

Dynamics of biologically available C and N in tundra communities.

WEINTRAUB, M.N.* and J.P.SCHIMEL

University of California, Santa Barbara, CA 93106 USA ¹

Abstract:
We characterized biologically available C and N in tundra soils by using long-term incubations, ¹⁴C and ¹⁵N isotope equilibration, and chemical fractionation. In tussock tundra soils, we found very little decline in respiration rate over 1 year, though soils respired almost 10% of the total C. This suggested that respiration was dominated by a large pool of potentially available substrate, rather than an 'active' fraction. In shrub soils there was a significant decline in respiration rates, indicating the presence of a smaller active C pool. To determine the nature of respired C, we used ¹⁴C isotope equilibration, in which microbial biomass was labeled by adding ¹⁴C glucose, and the isotope was equilibrated into the microbial biomass and products (MB&P) pool. By assaying the enrichment of both microbial biomass and CO₂, the size of the MB&P pool and proportion of respired C from MB&P can be estimated. In tussock, intertussock, and shrub soil, MB&P supplied 11%, 19%, 23% of the CO₂. In the wet meadow soil, MB&P supplied 26% of the CO₂ at the surface (0-5 cm), but only 6% 15-25 cm below the surface. MB&P represented 3% of soil C for tussock, 4% for inter-tussock, and 7% for shrub soil. In the wet meadow soil, MB&P comprised 5% of soil C at the surface, and 2% of the soil C in the horizons below. We have also fractionated the soils using a peat fractionation scheme. The best predictors of respiration were cellulose and lignin rather than soluble fractions. Thus, detritus chemistry was an important control on microbial activity, but ecosystem based differences were still more important: a straight correlation of respiration vs. ligno-cellulose index had an R² of 0.002.

Keywords: soil organic matter, C mineralization, N mineralization, isotope pool equilibration

This abstract is being presented at: 10:15 AM in session:
Oral Session #71: Soil Microbial Biomass and Soil Respiration.